There are many types of optical recording materials that are known. In many of the materials, the mode of operation requires that the unrecorded material have a high absorption and that the recorded areas, often referred to as pits, have low optical density or high reflection. The high reflection pits are made by ablating away the high absorption recording material, usually exposing an underlying reflective support.
One of the currently popular forms of optical storage of information is the compact disk or CD. Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background, the exact opposite of the above described optical recording materials. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce optical recording media which, when recorded in real time, produces a record that mimics the conventional CD on read out. Read out is at about 780 nm. In this manner, information can be added to the CD and the CD can be used on a conventional CD player.
One recently disclosed system of this type is the so called "Photo CD". In this system, conventional photographic film is first processed in a conventional manner. Then, the images from the film are digitized and the digital information is recorded in a CD readable form on an optical recording material. Images can then be played back on a CD type player into a conventional television. Since a CD has a capacity for a number of digitized images that is greater than the typical roll of consumer film, it is anticipated that the user will want to add images to a preexisting CD. Thus the need for recordable, CD compatible optical recording material.
Materials that can be used in this manner are described in U.S. Pat. No. 4,940,618, European Patent Application 0,353,393 and Canadian Patent Application 2,005,520.
Commercially useful materials of the type described in these references have stringent requirements. One of these requirements is light stability. Since the Photo CD is a consumer product, it must be capable of withstanding extreme environments. Between the time the original images are recorded on the Photo CD and the time subsequent images are recorded, the CD might be placed in strong sunlight, for example. The recording layer must be very light stable for this purpose.
In U.S. Pat. No. 4,626,496 there is described an optical recording medium wherein the recording layer consists essentially of a "double salt" of an organic dye cation and a metal complex anion. In the examples, most of the cationic dyes are cyanine dyes. The single charged anions are metal complexes, but no example is given of a doubly charged anion dye and in particular, there is no example of a doubly charged anionic azo dye. The main purpose for the anions in the '496 patent is to stabilize the cyanine dye. There is no disclosure of using the anion to make a index of refraction adjustment as well as stabilization. Based on the reported reflectivities, these "double salts" would not be useful in the writable CD type of system. This is shown in the comparative examples later in this specification.
Thus, there is a continuing need for optical recording materials that have the necessary optical characteristics so that they are CD compatible and yet are light stable. It is to a solution to this problem that the present invention is directed.